For the same reason that people used to have super fancy CVs and business cards and the like.
Back in the day? You were competing for jobs against the people in your town. Very few jobs involved people even moving across state lines, let alone cross country.
Now? We live in a global society. Even ignoring remote jobs, it is not horribly uncommon for people in more “technical” roles to move around the country or even the world for their career. And now you are competing against an entire country, if not planet, full of job candidates.
And that is where “building your brand” matters a lot. You need something that will make you stand out or make people remember who you are when they are reviewing CVs. I personally disagree with the idea of being a “hustle” person on linkedin, but I also know I got very lucky in when I was born as I have a pretty solid CV which opens a lot of doors for me. Whereas people even a few years younger than me need to fight hard to get past the filters and even get that first interview for a role.
And then you just have the act of keeping in feeds. Just making it a point to like posts and congratulate people on their work anniversaries means you have activity on your account which means you pop up in other people’s feeds. And I definitely know that I remembered the existence of an intern (who I actually really liked) because they congratulated a friend on a new job. Which led to me sending them a DM saying “hey, apply for this role”.